Abstract
ObjectivesTo explore the effects of computed tomography (CT) image characteristics and B-spline knot spacing (BKS) on the spatial accuracy of a B-spline deformable image registration (DIR) in the head-and-neck geometry.MethodsThe effect of image feature content, image contrast, noise, and BKS on the spatial accuracy of a B-spline DIR was studied. Phantom images were created with varying feature content and varying contrast-to-noise ratio (CNR), and deformed using a known smooth B-spline deformation. Subsequently, the deformed images were repeatedly registered with the original images using different BKSs. The quality of the DIR was expressed as the mean residual displacement (MRD) between the known imposed deformation and the result of the B-spline DIR.Finally, for three patients, head-and-neck planning CT scans were deformed with a realistic deformation field derived from a rescan CT of the same patient, resulting in a simulated deformed image and an a-priori known deformation field. Hence, a B-spline DIR was performed between the simulated image and the planning CT at different BKSs. Similar to the phantom cases, the DIR accuracy was evaluated by means of MRD.ResultsIn total, 162 phantom registrations were performed with varying CNR and BKSs. MRD-values < 1.0 mm were observed with a BKS between 10–20 mm for image contrast ≥ ± 250 HU and noise < ± 200 HU. Decreasing the image feature content resulted in increased MRD-values at all BKSs. Using BKS = 15 mm for the three clinical cases resulted in an average MRD < 1.0 mm.ConclusionsFor synthetically generated phantoms and three real CT cases the highest DIR accuracy was obtained for a BKS between 10–20 mm. The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels. Our results indicate that DIR accuracy in clinical CT images (typical noise levels < ± 100 HU) will not be effected by the amount of image noise.
Highlights
In radiotherapy, multiple volumetric images of a patient are often acquired to prepare and deliver a treatment plan with high accuracy in terms of dose and position
Treatment planning relies on computed tomography (CT) images whether or not in combination with positron emission tomography images and magnetic resonance imaging (MRI)
Phantoms with clinically realistic noise levels (1SD ≤ ± 100 HU) which were registered with B-spline knot spacing (BKS)-values between 8-20 mm, resulted in a mean residual displacement (MRD) < 1.0 mm
Summary
Multiple volumetric images of a patient are often acquired to prepare and deliver a treatment plan with high accuracy in terms of dose and position. In-room cone-beam CT (CBCT) images are often acquired for treatment position verification. These CBCTs may potentially be used for treatment evaluation and adaptation. For head-and-neck cancer patients, significant changes in the patient anatomy between the reference situation (e.g. planning CT) and the course of treatment can gradually occur and are mainly related to weight loss, tumor regression and resolution of edema [1,2]. As the head-and-neck anatomy is characterized by several vulnerable normal tissues, often in the vicinity of steep dose gradients, monitoring of these anatomical changes becomes increasingly important. Adaptive radiotherapy protocols are used to correct for anatomic and morphologic changes during treatment, aiming at improved local tumor control and/or a reduction of radiation induced side effects
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